Superdirective antennas typically comprise two or more radiating elements in close proximity (the spacing of the radiating (or receiving) elements is <λ/4, where λ is the wavelength of the signal to be radiated and/or received by the antenna).
Antenna arrays are used in numerous applications: communications, radar, signal intelligence, etc. Perhaps the most attractive features of antenna arrays are beam-synthesis and reconfigurability. For example, phased arrays have one or more beams that may be reconfigured to point in different directions or have different beam characteristics by changing the weight (phase and/or amplitude) applied to the signal at each antenna element. In digital beamforming arrays, the signal may be recorded independently at each element, and beams may be formed in post processing. Multiple-Input and Multiple-Output (MIMO) technology, known in the art, can be important in wireless communications systems since because it offers data throughput improvements without using additional bandwidth or increasing transmit power.
Array synthesis techniques are available in the literature that show how to a) increase the directivity of the array without increasing the physical size and b) generate nulls in the radiation pattern that will provide immunity to interfering or jamming signals. However, these techniques have severe limitations in real arrays due to mutual coupling. Specifically, it is well known that prior art superdirective antenna arrays have a high Q, and therefore suffer from a corresponding efficiency/bandwidth limitation. Due to this limitation, superdirective antenna arrays are widely regarded as problematic and are not widely deployed. This invention reduces the Q of superdirective antennas by more than 10 times, providing greater than a 10 dB improvement in the realized gain (RF efficiency) of superdirective antennas. This reduction in Q is also helpful in generating pattern nulls.
Electrically small antennas are antennas which are rather small (or short) compared to the wavelengths of the radio frequencies they are intended to receive. Conventional full length antennas are typically a′4 or ½ wavelength in size. At the frequencies used for some handheld device applications, antennas which are much smaller are called for. Electrically small antennas can be defined as antennas whose elements are 1/10 (or less) of a wavelength of the radio frequencies they are intended to receive. Electrically small antennas also tend to have high Qs, so they tend to have a small bandwidth compared to conventional antennas.
The prior art may include:
Passive Superdirective Arrays:
There is plentiful academic work (starting with Oseen in 1922) that reveals the difficulty of realizing significant bandwidth and efficiency. Two key conclusions are that optimum directivity leads to extremely high Q and that mutual coupling makes for difficult feed network design. Few arrays have been realized, and these arrays have efficiencies <−20 dB. The practical limitations are:
(1) High Antenna Q small bandwidth;
(2) Low radiation resistance low efficiency; and
(3) Tight tolerances difficult to realize feed network.
For a paper on the subject, see R. C. Hansen, “Fundamental Limitations in Antennas,” Proceedings of the IEEE, v. 69, no. 2, February 1981.
The Use of Metamaterials Placed Between Radiating Elements to Decouple them:
See, for example, K. Buell, et al. “Metamaterial Insulator Enabled Superdirective Array,” IEEE Trans. Antenn. Prop., April, 2007. The disadvantages of this approach are:
(1) Narrow bandwidth;
(2) Only applicable to printed antennas;
(3) Complicated fabrication; and
(4) Not easily tuned.
Active Antennas:
Directly feed antennas with transistor active impedance matching networks. This works because transistor active component inputs and outputs are approximated by open circuits and hard sources, respectively. Therefore, mutual coupling has no effect. However, the antennas are not matched, resulting in low receiver sensitivity and low transmit efficiency. For example, see M. M. Dawoud and A. P. Anderson, “Superdirectivity with appreciable Bandwidth in Arrays of Radiating Elements Fed by Microwave Transistors,” European Microwave Conference, 1974.
Digital Beamforming:
An analog-to-digital converter at each antenna element digitizes the signal so that arbitrary beams may be formed in the digital domain. In addition, mutual coupling can be accounted for in the beamforming (see C. K. Edwin Lau, Raviraj S. Adve, and Tapan K. Sarkar, “Minimum Norm Mutual Coupling Compensation With Applications in Direction of Arrival Estimation,” IEEE Transactions on Antennas and Propagation, Vol. 52, No. 8, August 2004, pp. 2034-2041). However, the physical impedance match is only valid for a single radiation pattern, which results in limited receive sensitivity for other patterns. Furthermore, high resolution and high dynamic range analog-to-digital converters are needed to realize superdirective patterns.
Decoupling Networks:
Decoupling Networks result in independent modes with orthogonal patterns from the antenna array. These modes can be matched independently and used to synthesize arbitrary radiation patterns. However, this approach does not reduce antenna Q. For reference, see Christian Volmer, Metin Sengül, Jörn Weber, Ralf Stephan, and Matthias A. Hein, “Broadband Decoupling and Matching of a Superdirective Two-Port Antenna Array, IEEE AWPL, vol. 7, 2008.
Multimode Antenna Structure:
This technology connects nearby antennas with conductors to decouple them. The approach is narrowband and alters the radiation modes of the structure. Furthermore, seems to only be applicable to small numbers of elements. See U.S. Pat. No. 7,688,273.
Non-Foster Matching Circuits for Single Antennas:
See the following documents and the comment below:
    S. E. Sussman-Fort and R. M. Rudish, “Non-Foster impedance matching of electrically-small antennas,” IEEE Trans. Antennas Propagat., vol. 57, no. 8, August 2009.    J. G. Linvill, “Transistor Negative Impedance Converters,” Proc. IRE, vol. 41, no. 6, pp. 725-729, June 1953.
This prior art technology pertains to single antennas rather than to antenna arrays.
Non-Foster Matching Circuits Connected in Series with Array Elements or Between Dipole Ends in Large Arrays:
See the following documents and the comments below:
    (1) S. E. Sussman-Fort and R. M. Rudish “Progress in use of non-Foster impedances to match electrically-small antennas and arrays,” Antenna Applications Symposium Digest, 2005.    (2) R. C. Hansen, “Wideband Dipole Arrays Using Non-Foster Coupling,” Microwave and Optical Technology Letters, 38(6), Sep. 20, 2003, pp. 453-455.    (3) Applicable to large arrays, not to superdirectivity. Calculations are valid for conventional phased array scanning.    (4) Does not match all modes simultaneously.
Superdirectivity has been sought after for 90 years, and is still regarded as impractical due to the resulting high antenna Q. The prior art in superdirectivity is not capable of reducing the antenna Q. Previous approaches produce either narrow-band results or low efficiency.